I’ve been a few times out to this area, because it is really interesting and steeped in History.
What first started my research was the discovery that there were a number of Tumulus in the area, so with a tip from a friend I went looking and found 3 easily..
Kofun (ancient mounds) refers to the huge burial mounds built in Yamato after the middle of the third century, however I think these are later, according to the Otawara city website there are several Samurai mounds dating to 1692
The Nakagawa fish park is in this area, this contains a major aquarium with many rare fish and an Amazon basin Reconstruction . I haven’t been yet, but its on my list.
Also the Horin ji temple is in the area. I’m still trying to find out more about that, but every search brings up a temple of the same name in Kyoto (that is much more significant!)
I’m sure there is a secret temple website somewhere that I don’t know about :-), I really struggle to find anything about any of the temples… Anyway, I will keep looking